Semiconductor device with a reduced band gap and process

ABSTRACT

The application relates to a semiconductor device made of silicon with regionally reduced band gap and a process for the production of same. One embodiment provides a semiconductor device including a body zone, a drain zone and a source zone. A gate extends between the source zone and the drain zone. A reduced band gap region is provided in a region of the body zone, made of at least ternary compound semiconductor material.

BACKGROUND

The application relates to a semiconductor device made of silicon withregionally reduced band gap and a process for the production of same.

A semiconductor device made of silicon with a regionally reduced bandgap is known from PCT/US2005/036036 which discloses a transistor withMOS-gate including a body region with a source region and a drain regionwhich forms a pn-junction with the body region, the source having alower energy gap than the body region. The lower energy gap in thesource region is achieved due to the fact that, although the source zoneremains highly n-doped, it consists of a binary compound semiconductorcontaining silicon and germanium and thus provides a band gap for thesource zone which is smaller than the silicon band gap but larger thanthe germanium band gap. This weakens a parasitic bipolar transistor andthus improves the avalanche behaviour of the semiconductor device.

The production of a semiconductor device with a band gap in the sourcezone presents significant difficulties to be resolved in that followingproduction of the source zone further high temperature processes arestill required to produce a MOS gate transistor of this type. However,these high temperature processes create a risk that germanium willdiffuse out of the silicon lattice in processes above 600° C., therebyrendering it impossible to maintain the desired reduced band gap in thesource zone.

Other solutions for reducing the flow voltage drop of the body diodeoccurring at the pn-junction between the body zone and the drain zonewhich are known from the prior art are based on the parallel connectionof an additional diode with a lower flow voltage parallel to the bodydiode. These parallel-connected diodes with lower flow voltage caneither be connected externally in the form of germanium diodes orintegrated in the semiconductor chip in the form of Schottky diodes.However, integrated Schottky diodes reduce the semiconductor areaavailable for the MOSFET and the use of Schottky transitions results ina higher area-specific closing resistance.

In addition to the aforementioned germanium diodes of lesser band gapwhich can be connected externally in parallel to the body/drainpn-junction, it is also possible to integrate germanium diodes of thistype with a typical flow voltage of 0.2 to 0.3 V on a silicon chip.However, germanium diodes integrated in this manner would have arelatively high leakage current when blocked, and due to the small bandgap operating temperatures would have to be limited to below 90° C.,temperatures unacceptable for power semiconductor devices. In addition,the leakage currents of pure germanium diodes increase exponentially attemperatures above 50° C.

Integrated diodes made of an SiGe binary compound semiconductor alsopresent certain disadvantages relating to the thicknesses of the variouslayers needed for a silicon-germanium diode which necessarily require abuffer layer if SiGe is to be grown on a monocrystalline silicon crystalregion, for example, in order to avoid crystal defects and to switch thelattice constants of the monocrystalline silicon to the latticeconstants of the binary compound semiconductor SiGe. If this bufferlayer is accommodated in the body region, for example, the flow voltageof the body diode increases. At the same time, the thickness of theuseful SiGe layer is limited and too low for the layer thicknessesrequired for power semiconductor devices with an integrated SiGe diodemade of a binary compound semiconductor material.

For these and other reasons, there is a need for the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the present invention and are incorporated in andconstitute a part of this specification. The drawings illustrate theembodiments of the present invention and together with the descriptionserve to explain the principles of the invention. Other embodiments ofthe present invention and many of the intended advantages of the presentinvention will be readily appreciated as they become better understoodby reference to the following detailed description. The elements of thedrawings are not necessarily to scale relative to each other. Likereference numerals designate corresponding similar parts.

FIG. 1 illustrates a schematic partial cross-section of one embodimentof a semiconductor device.

FIGS. 2 to 11 illustrate schematic cross-sections of part views ofdevices in the production of one embodiment of a semiconductor devicemade of silicon with regionally reduced band gap as illustrated in FIG.1.

FIG. 2 illustrates a schematic cross-section through a part of asemiconductor wafer made of monocrystalline silicon which is used as thesubstrate for the semiconductor device illustrated in FIG. 1.

FIG. 3 illustrates a schematic cross-section through the part of thesemiconductor wafer illustrated in FIG. 2 with a fully grown siliconepitaxy layer.

FIG. 4 illustrates a schematic cross-section through the partillustrated in FIG. 3 after the introduction of a p-conducting bodyzone.

FIG. 5 illustrates a schematic cross-section through the partillustrated in FIG. 4 after the introduction of an n⁺-conducting sourcezone.

FIG. 6 illustrates a schematic cross-section through the partillustrated in FIG. 5 after the etching in of trench structures.

FIG. 7 illustrates a schematic cross-section through the partillustrated in FIG. 6 after the application of a gate oxide layer.

FIG. 8 illustrates a schematic cross-section through the partillustrated in FIG. 7 after the partial filling of the trench structurewith an electrically conductive material.

FIG. 9 illustrates a schematic cross-section through a part illustratedin FIG. 8 after the application of an insulating intermediate layer andthe introduction of a source contact window into the insulatingintermediate layer.

FIG. 10 illustrates a schematic cross-section through a part illustratedin FIG. 9 after the etching of a source contact opening through thesource zone to project into the body zone.

FIG. 11 illustrates a schematic cross-section through a part illustratedin FIG. 10 after the deposition of a ternary compound semiconductor inthe base region of the source contact opening.

FIG. 12 illustrates a schematic partial cross-section of one embodimentof a semiconductor device.

FIG. 13 illustrates a schematic partial cross-section of one embodimentof a semiconductor device.

FIG. 14 illustrates a schematic partial cross-section of one embodimentof a semiconductor device of a further embodiment.

FIG. 15 illustrates a schematic partial cross-section of one embodimentof a semiconductor device.

DETAILED DESCRIPTION

In the following Detailed Description, reference is made to theaccompanying drawings, which form a part hereof, and in which is shownby way of illustration specific embodiments in which the invention maybe practiced. In this regard, directional terminology, such as “top,”“bottom,” “front,” “back,” “leading,” “trailing,” etc., is used withreference to the orientation of the Figure(s) being described. Becausecomponents of embodiments of the present invention can be positioned ina number of different orientations, the directional terminology is usedfor purposes of illustration and is in no way limiting. It is to beunderstood that other embodiments may be utilized and structural orlogical changes may be made without departing from the scope of thepresent invention. The following detailed description, therefore, is notto be taken in a limiting sense, and the scope of the present inventionis defined by the appended claims.

One embodiment of the present application relates to a semiconductordevice made of silicon with regionally reduced band gap and a processfor the production of same. The semiconductor device made of silicon hasa body zone which forms a pn-junction with a drain zone. A source zonewith the same type of conductivity as the drain zone is positioned inthe region of the body zone. A gate extends between the source zone andthe drain zone. A source electrode positioned in a source contactopening projects into the body zone. In this arrangement the sourcecontact opening has in the region of the body zone beneath the sourceelectrode a zone of at least ternary compound semiconductor materialwith a band gap smaller than the band gap of silicon.

With this semiconductor device, due to the regionally reduced band gapin the body zone the flow voltage of the body pn-junction is lower thanis the case with a transistor made of monocrystalline silicon. Further,with a third component in the ternary compound semiconductor material itis possible to reduce crystal stresses due to the different latticeconstants of silicon and a merely binary compound semiconductormaterial, thereby reducing crystal defects caused by lattice strain evenwith thicker deposition layers.

In a ternary compound semiconductor material based on the binarycompound semiconductor material SiGe, lattice strains can be reduced byadding quadrivalent carbon to substitutional lattice sites in such amanner that it is possible to create an almost voltage-free transitionfrom an area with the band gap of the silicon to a region with reducedband gap, thereby not only reducing the flow voltage, but alsoeliminating the need for buffer layers.

FIG. 1 illustrates a schematic cross-section of one embodiment of asemiconductor device 1. The semiconductor device 1 is a MOS-transistorand consists of a semiconductor body 24 containing monocrystallinesilicon which has a large-surface drain electrode D on its back 25. Ann-conducting epitaxy layer 19 is applied to an nonconducting substrate15 and the top 26 of the semiconductor body 24 is provided with a trenchgate structure 27, mesa-structures 28 and trench structures 29alternating with the trench gate structure 27.

In the mesa-structures 28 a p-conducting body zone 6 is integrated intothe n-conducting silicon-epitaxy layer. In the vicinity of the top 26 ofthe semiconductor body 24 the body zone 6 turns into a n⁺-conductingsource zone. A source contact opening 12 is etched to project throughthe source zone 9 and into the body zone 6 in the mesa structure 28. Aregion with a smaller band gap than silicon is created in a base region30 of this source contact opening 12 by positioning a ternary compoundmaterial made of SiGeC in the base region 30.

In this region of the ternary compound material 13 it is possible toincrease the dopant concentration gradually or abruptly toconcentrations of the order of 10¹⁸ cm³ dopant atoms per cubiccentimetre starting from the concentration of the body zone up to thetransition to the electrically conducting source electrode 11. With thisregion which is made of a ternary compound semiconductor material 13 itis possible to reduce the flow voltage at the pn-junction 8 from thep-conducting body zone to the n-conducting drain zone.

Furthermore, with the additional carbon atoms on substitutional latticesites it is possible to partially compensate for the lattice strainscaused by the mismatch between silicon and germanium with carbon, suchthat this layer of ternary compound semiconductor material 13 has areduced crystal defect density in the source contact opening. Thus it ispossible to insert a layer thicker than the one illustrated here intothis type of source contact opening. Moreover, in one embodiment theintroduction of the ternary compound semiconductor 13 takes place aftercompletion of the high temperature processes in the production of thesemiconductor device 1 so that the germanium content is maintainedrather than being reduced.

In one embodiment, the effect of a carbon concentration C in the ternarycompound semiconductor material 13 can be achieved with a ratio tosilicon and germanium of between 1:500≦C≦1:20. Simulated calculationshave illustrated that with a carbon concentration C in the ternarycompound semiconductor material 13 with a ratio to silicon and germaniumof between 1:200≦C≦1:100, optimum reduction of lattice stresses ispossible and that at the same time the forward voltages decisive for theoperation of the semiconductor device 1 are reduced in relation to puresilicon.

In this arrangement the germanium concentration in the ternary compoundsemiconductor 13 is relatively high in relation to silicon and carbon,lying between 1:10≦Ge≦1:1. An optimum germanium concentration in theternary compound semiconductor material 13 in relation to silicon andcarbon has proved to be between 1:5≦Ge≦1:4. A clear reduction in voltageis achieved at this germanium concentration and both crystal defects andleakage currents have optimally low values.

The region with the ternary compound semiconductor material 13 in thebody zone 6 is more highly doped than the body zone 6 itself. In oneembodiment, the dopant gradient increases gradually from the body zonedoping p within the ternary compound semiconductor materials 13 towardsthe source electrode 11 and reaches its highest value close to theaforementioned concentration of 10¹⁸ cm³ dopant atoms per cubiccentimetre immediately beneath the source electrode 11.

The ternary compound semiconductor material 13 is inserted into a sourcecontact opening 12, as illustrated in FIG. 1, and as a result there areno further high temperature processes for the production of the siliconsemiconductor device 1 after the introduction of the ternary compoundsemiconductor material 13. Nevertheless, it is possible due to theternary content to ensure that the subsequent process temperatures canbe set several hundreds of degrees higher than those for semiconductordevices with binary compound materials made of germanium and siliconbefore any serious out-diffusion of the germanium takes place. Thistemperature is approximately 800° C.

In summary, it can be said that the structure may provide the following:

1. There is no longer any need to limit the layer thickness of theternary compound semiconductor material.

2. A buffer layer of silicon and germanium as required for binarycompound semiconductor materials is no longer needed.

3. Higher temperatures of up to approximately 800° C. are possible forsubsequent processes.

4. The special properties of the regionally smaller band gap of binarySiGe remain unchanged despite the substitutional integration of C.

A process for the production of a semiconductor device 1 with regionallyreduced band gap is illustrated in FIGS. 2 to 11 below and includes thefollowing process steps. First, as illustrated in FIG. 1, asemiconductor wafer 14 made of monocrystalline silicon is provided asthe semiconductor substrate 15. In this embodiment, the semiconductorsubstrate is n⁺-conducting and has a back 25 and a front 31.

FIG. 3 illustrates a schematic cross-section through the part of thesemiconductor body 24 illustrated in FIG. 2 with a fully grownsilicon-epitaxy layer 19. This silicon-epitaxy layer 19 is n-conductingand largely forms a drift path 32 of a drain zone 7.

FIG. 4 illustrates a schematic cross-section through the part of thesemiconductor wafer 14 illustrated in FIG. 3 after the introduction of ap-conducting body zone 6 with a complementary type of conductivity intothe n-conducting epitaxy layer which represents a drift zone 32 of thedrain zone 7.

FIG. 5 illustrates a schematic cross-section through the partillustrated in FIG. 4 after the application of a n⁺-conducting sourcezone 9 to the p-conducting body zone 6. These different zones can beapplied to large surface areas of the semiconductor wafer 14 for thesemiconductor body 24 as long as no charge compensation zones areprovided for this MOS transistor in drift path 32.

FIG. 6 illustrates a schematic cross-section through the partillustrated in FIG. 5 after the etching in of trench structures 33 forthe trench gate structure to be applied subsequently. To this end, asillustrated in FIG. 7, a gate oxide layer 20 is applied to the structureillustrated in FIG. 6. This gate oxide layer 20 is applied by thermaloxidation using pure oxygen in order to create a silicon oxide layerwhich is as ion-free as possible in a thickness of between 90 and 300nanometres. However, this oxide layer is required only in the region ofthe trench structure 33 and parts of it are removed during later etchingprocesses and at the latest when the source contact windows are opened.

FIG. 8 illustrates a schematic cross-section through the partillustrated in FIG. 7 after the partial filling of the trench structurewith an electrically conductive material 18 which is highly dopedpolysilicon and forms the gate electrodes 10 of the semiconductordevice.

FIG. 9 illustrates a schematic cross-section through a part illustratedin FIG. 8 after the application of an insulating intermediate layer 21and the introduction of a source contact window 34 into the insulatingintermediate layer 21 which in this embodiment can be a silicon oxide. Asource contact opening is then etched into the silicon material of themesa structure 28 in the region of the source contact window 34 in thedirection of arrow A, a dry etching technique with a correspondingreactive plasma being used to carry out an anisotropic etching processin the direction of arrow A.

FIG. 10 illustrates a schematic cross-section through the partillustrated in FIG. 9 dafter the etching of a source contact openingthrough the source zone 9 and projecting into the body zone 6, thesource contact opening 12 being pushed as close as possible to thepn-junction 8 to achieve a parallel connection to the body zone in orderto reduce the flow voltage of the body zone pn-junction 8.

FIG. 11 illustrates a schematic cross-section through a part illustratedin FIG. 10 after deposition of a ternary compound semiconductor 13 inthe base region 30 of the source contact opening 12. Thus, followingcompletion of the high temperature processes, a sequence of layers 17made of a ternary compound semiconductor material 13 is positioned in alower region of the source contact opening 12. The source contactopenings 12 can then be filled with electrically conducting material fora source electrode 11, as illustrated in FIG. 1.

This production process, in which the deposition of a ternary compoundsemiconductor 13 in the form of SiGeC is provided relatively far intothe process for producing a semiconductor device structure and theternary compound semiconductor material is inserted only in the regionof the open contact hole trench, has the advantage that no further hightemperature processes are required to act on the device since gateoxidation, where appropriate field oxidation, the application of aninsulating intermediate layer 21 made of silicon dioxide, etc. havealready taken place. The source contact opening 12 can be etched asdeeply as possible in relation to the lower pn-junction 8 between thebody zone 6 and the drain zone 7 using an etching technique, a dryetching technique, before the doped ternary compound semiconductormaterial 13 is deposited to fill the source contact hole.

In order to produce at least the structures illustrated in FIG. 12, thesequence of process steps described above is modified in that the trenchstructures are inserted before the body and source zones and additionalprocess steps are required to insert both field plates with a fieldoxide and gate electrodes with a gate oxide into a trench structure as atrench gate structure. It is also to etch an appropriate trenchstructure into the semiconductor wafer even in the case of semiconductordevices which have only one trench gate structure.

Thus a process for the production of a semiconductor device withregionally reduced band gap includes the following process steps. First,a silicon semiconductor wafer is provided as the semiconductorsubstrate. A trench structure for receiving at least one MOS trench gatestructure is inserted in the semiconductor wafer. Following introductionof the semiconductor structures into the silicon semiconductor waferwhich has at least one body zone, one drain zone, one MOS trench gateand one source zone, source contact openings are produced which projectthrough the source zone into the body zone and terminate shortly beforea pn-junction to a drain zone positioned beneath it. Followingcompletion of the high temperature processes, a sequence of layers madeof a ternary compound semiconductor material are deposited in a lowerregion of the source contact openings, after which the source contactopenings are filled with electrically conducting material for a sourceelectrode.

If a field plate structure is also to be inserted into the trenchstructure, the process for producing a semiconductor device withregionally reduced band gap includes the following process. A siliconsemiconductor wafer is provided as the semiconductor substrate. A trenchstructure for receiving at least one field plate structure and one MOStrench gate structure is inserted into the semiconductor wafer.Following the introduction of the semiconductor structures into thesilicon semiconductor wafer which has at least one body zone, one drainzone, one field plate, one MOS trench gate and one source zone, sourcecontact openings are produced which project through the source zone intothe body zone and terminate shortly before a pn-junction positionedbeneath it. Following completion of the high temperature processes, asequence of layers made of a ternary compound semiconductor material aredeposited in a lower region of the source contact opening and the sourcecontact openings are then filled with electrically conducting materialfor a source electrode.

In one embodiment, the sequence of layers 17 made of the ternarycompound semiconductor material 13 is produced in the lower region ofthe source contact opening 12 by using chemical or physical gas phasedeposition. However, it is also possible to use an MBE Molecular BeamEpitaxy process or to attempt to fill the source contact openings 12with the ternary compound semiconductor material 13 by ion implantationfollowed by recrystallisation. MBE processes for power semiconductordevices are, however, different in that the required thicknesses havecost, material and time implications. Given the high dosages ofgermanium and carbon required, ion implantation would lead to anamorphization of the silicon which it would not be possible to cure withthe possible after-treatment temperatures of up to 800° C. As a resultthe dopant can not be sufficiently activated.

The dopant to be inserted during the deposition of the ternary compoundsemiconductor material 13 is inserted in a higher dopant concentrationthan the dopant concentration present in the body zone 6. Here it ispossible to allow the dopant concentration to increase gradually fromthe dopant concentration of the body zone 6 up to the transition to thesource electrode 11.

FIG. 12 illustrates a schematic cross-section through a part of asemiconductor device 2 as disclosed in a further embodiment. Componentswith functions identical to those in the preceding figures aredesignated using the same reference numerals and are not discussed ingreater detail here. In this embodiment, too, a trench gate electrode 10is integrated, the trench being deepened in such a manner that it isalso possible to position in the trench a field plate 22 which isinsulated from the drift path by a field oxide 23. However, the sourcepotential to which the field plate 22 is set means that the fielddistribution in the drift path 32 can be influenced in such manner thatthe drift path can be more highly doped, and thus it is possible toachieve a more favourable closing resistance for the whole semiconductordevice 2 with vertical field plates 22.

Independently of this field plate structure, however, a zone with aternary compound semiconductor material 13 once again projects into thebody zone 6, the ternary compound semiconductor material having asmaller band gap than the surrounding silicon, thereby reducing the flowvoltage of the body diode in this semiconductor device 2.

FIG. 13 illustrates a schematic cross-section through a part of asemiconductor devices 3 as disclosed in a further embodiment, this parthaving several transistor cells of a MOS transistor with a trench gatestructure. A region with a ternary compound semiconductor material 13 ispositioned in the body zone 6 of each of the cells in order to reducethe flow voltage of the body zone pn-junction.

FIG. 14 illustrates a schematic cross-section through a part of asemiconductor device 4 as disclosed in a further embodiment. Thisembodiment differs from the previous version in that a lateral gatestructure is provided, a region with a reduced band gap consisting of asequence of layers 17 of a ternary compound semiconductor materials 13once again projecting into the body zone 6 and being positioned beneaththe source electrodes 11.

FIG. 15 illustrates a schematic cross-section through a part of asemiconductor device 5 as disclosed in a further embodiment. Thisembodiment differs from the preceding versions in that the previousversions illustrate vertical MOS transistors while here there is alateral MOS transistor, a sequence of layers 17 of a ternary compoundsemiconductor material which has a lower band gap than the surroundingsilicon once again projecting into the body zone 6. In a lateral MOStransistor source S, gate G and drain D are positioned on the top 26 ofthe semiconductor body 24 and the drift path 32 extends laterallybetween the body zone 6 and the drain electrode D.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat a variety of alternate and/or equivalent implementations may besubstituted for the specific embodiments shown and described withoutdeparting from the scope of the present invention. This application isintended to cover any adaptations or variations of the specificembodiments discussed herein. Therefore, it is intended that thisinvention be limited only by the claims and the equivalents thereof.

1. A semiconductor device comprising: a body zone, a drain zone and asource zone; a gate which extends between the source zone and the drainzone; and a reduced band gap region in a region of the body zone,comprising of at least ternary compound semiconductor material; whereina lateral extent of the zone of ternary compound semiconductor materialcorresponds to the lateral extent of a source contact opening.
 2. Thesemiconductor device of claim 1, wherein the ternary compoundsemiconductor material comprises silicon, germanium and carbon.
 3. Thesemiconductor device of claim 1, wherein the ternary compoundsemiconductor material is more highly doped than the body zone.
 4. Thesemiconductor device of claim 1, wherein a metal of the source contactis directly in contact with the ternary compound semiconductor material.5. A semiconductor device made of silicon comprising: a body zone; adrain zone which forms a pn-junction with the body zone; a source zonewith the same type of conductivity as the drain zone, the source zonebeing positioned in the region of the body zone; a gate which extendsbetween the source zone and the drain zone; a source electrode which ispositioned in a source contact opening and projects into the body zone;and the source contact opening in the region of the body zone beneaththe source electrode having a zone of at least ternary compoundsemiconductor material with a band gap which is smaller than the bandgap of silicon; wherein a lateral extent of the zone of ternary compoundsemiconductor material corresponds to the lateral extent of the sourcecontact opening.
 6. The semiconductor device of claim 5, wherein theternary compound semiconductor material comprises silicon, germanium andcarbon.
 7. The semiconductor device of claim 6, wherein the carbonconcentration C in the ternary compound semiconductor material isbetween 1:500≦C≦1:20 in relation to the sum of the silicon andgermanium.
 8. The semiconductor device of claim 6, wherein the carbonconcentration C in the ternary compound semiconductor material isbetween 1:200≦C≦1:100 in relation to the sum of the silicon andgermanium.
 9. The semiconductor device of claim 6, wherein the germaniumconcentration in the ternary compound semiconductor material is between1:10≦Ge≦1:1 in relation to the sum of the silicon and carbon.
 10. Thesemiconductor device of claim 6, wherein the germanium concentration inthe ternary compound semiconductor material is between 1:5≦Ge≦1:4 inrelation to the sum of the silicon and carbon.
 11. The semiconductordevice of claim 5, wherein the ternary compound semiconductor materialis more highly doped than the body zone.
 12. A semiconductor devicecomprising: a body zone, a drain zone and a source zone; a gate whichextends between the source zone and the drain zone; and a reduced bandgap region in a region of the body zone, comprising of at least ternarycompound semiconductor material, wherein the doping level of the zonecomprising the ternary compound semiconductor material is varied suchthat the doping level is higher directly adjacent a metal of a sourcecontact.
 13. A semiconductor device made of silicon comprising: a bodyzone; a drain zone which forms a pn-junction with the body zone; asource zone with the same type of conductivity as the drain zone, thesource zone being positioned in the region of the body zone; a gatewhich extends between the source zone and the drain zone; a sourceelectrode which is positioned in a source contact opening and projectsinto the body zone; and the source contact opening in the region of thebody zone beneath the source electrode having a zone of at least ternarycompound semiconductor material with a band gap which is smaller thanthe band gap of silicon; wherein the doping level of the zone comprisingthe ternary compound semiconductor material is varied such that thedoping level is higher directly adjacent a metal of the sourceelectrode.